Best Practice 6: Confirming That the Critical Path Is Valid
The critical path is generally defined as the longest continuous sequence of activities in a schedule. It defines the program’s earliest completion date or minimum duration. Activities on this path are termed “critical path activities.” Typically, the sequence of activities with the longest total duration is also the path through the network with the lowest total float. As Best Practice 7 shows, total float is the time an activity can slip before its delay affects the program end date. When the network is free of date constraints, critical activities have zero float, and therefore any delay in the critical activity causes the same day-for-day delay in the program forecast finish date.
For example, if an activity on the critical path is delayed by a week, the program finish date will be delayed by a week unless the slip is mitigated. Therefore, the critical path is most useful as a tool to help determine which activities deserve focus and, potentially, management help. The critical path assists program management in prioritizing resources to have the most positive effect on program performance.23
In figure 28, the critical path is made up of three detail work activities: “install carpeting and wood flooring,” “install kitchen cabinets and countertops,” and “set plumbing fixtures.” The activities are 6 working days, 2 working days, and 3 working days long, respectively. The minimum duration of this particular sequence of activities is therefore 11 days. An additional day’s activity, “install laundry room cabinets and countertops,” is performed in parallel with “install kitchen cabinets and countertops”—that is, the installation of cabinets and countertops in both the laundry room and the kitchen may start after the installation of the carpeting and wood flooring finishes.
Figure 28: The Critical Path and Total Float
When cabinets and countertops have been installed in the kitchen and laundry room, the plumbing fixtures can be set, and then the interior finishing of the house is considered complete. Because “install laundry room cabinets and countertops” is 1 day shorter than “install kitchen cabinets and countertops,” it does not directly affect the start date of the “interior finishes complete” milestone. Notice that “install laundry room cabinets and countertops” has an early start of January 16 and a late start of January 20. In other words, laundry room cabinet and countertop installation could start as late as January 20 (1 working day later than planned) and, if finished in 1 day, would have no effect on the “interior finishes complete” milestone date of January 23.
The difference between early and late dates for “install laundry room cabinets and countertops” yields 1 working day of total float. The early and late start dates for “install kitchen cabinets and countertops,” however, are equal. If the kitchen cabinet installation is delayed by 1 day, or if its duration extends by 1 day, then the “interior finishes complete” milestone slips by 1 day. “Install kitchen cabinets and countertops” has zero float and is on the critical path.
Scheduling software automatically calculates a critical path through a network of activities by defining as critical the activities that have less than predefined total float. Typically, total float is set to zero, and the scheduling software marks as critical all activities with zero or less-than-zero total float.
Activities with total float within a narrow range of the critical path total float are “near-critical” because they can quickly become critical if their float is used up in a delay. Near-critical paths need only a small extension of time to become critical. Management must monitor critical and near-critical activities through sound schedule management because any delay in them will delay the entire program. Near-critical paths are monitored according to a float threshold tailored to the program. For example, a brief schedule might consider a 5-day slip to be a near-critical threshold. In programs scheduled to take years, a 2- or 3-month’s slip in near-critical paths might make the path critical. Because prolonging a schedule by 5 days on a short project is as easily possible as prolonging a multiyear project several months, program managers should manage all near-critical and critical paths.
The critical path is not constant. The sequence of activities that make up the critical path changes as activities are delayed, finished early, occur out of planned sequence, and so on. Activities that were previously critical may become noncritical, and activities that were not critical may become critical.
It is crucial that program management understand that an important activity is not necessarily “critical.” At any point in time, the critical path may or may not contain activities that management believes are particularly important. A delay in an activity may be important for any number of reasons related to scope and cost without delaying the finish milestone date. In contrast, some mundane activities—training, for example—may be on the critical path and not particularly risky but can delay the program finish date if they take longer to accomplish. Similarly, an activity of long duration should not be referred to as a “critical path activity” simply because it will take a long time to accomplish. “Critical activity” in scheduling parlance has a specific definition that should be adhered to when reporting and evaluating schedule data.
In this section, we discuss the deterministic critical path—that is, the path as defined by the initial or currentset of inputs in the schedule model. However, the true critical path of a schedule is uncertain becausedurations of activities are uncertain. Best Practice 8 discusses a probabilistic or risk critical path that is based on assumptions about estimating error and risk.↩︎